1. Field of the Invention
The present invention relates generally to the field of surgical tissue removal systems, and more specifically to modulated irrigation and aspiration during surgical procedures such as phacoemulsification.
2. Description of the Related Art
Phacoemulsification surgery has been successfully employed in the treatment of certain ocular problems, such as cataracts. Phacoemulsification surgery utilizes a small corneal incision to insert the tip of at least one phacoemulsification handheld surgical implement, or handpiece. The handpiece includes a needle which is ultrasonically driven once placed within an incision to emulsify the eye lens, or break the cataract into small pieces. The broken cataract pieces may subsequently be removed using the same handpiece or another handpiece in a controlled manner. The surgeon may then insert lens implants in the eye through the incision. The incision is allowed to heal, and the results for the patient are typically significantly improved eyesight.
As may be appreciated, the flow of fluid to and from a patient through a fluid infusion or extraction system and power control of the phacoemulsification handpiece is critical to the procedure performed. Different medically recognized techniques have been utilized for the lens removal portion of the surgery. Among these, one popular technique is a simultaneous combination of phacoemulsification, irrigation and aspiration using a single handpiece. This method includes making the incision, inserting the handheld surgical implement to emulsify the cataract or eye lens. Simultaneously with this emulsification, the handpiece provides a fluid for irrigation of the emulsified lens and a vacuum for aspiration of the emulsified lens and inserted fluids.
Currently available phacoemulsification systems include a variable speed peristaltic pump, a vacuum sensor, an adjustable source of ultrasonic power, and a programmable microprocessor with operator-selected presets for controlling aspiration rate, vacuum and ultrasonic power levels. A phacoemulsification handpiece is interconnected with a control console by an electric cable for powering and controlling the piezoelectric transducer. Tubing provides irrigation fluid to the eye and enables withdrawal of aspiration fluid from an eye through the handpiece. The hollow needle of the handpiece may typically be driven or excited along its longitudinal axis by the piezoelectric effect in crystals created by an AC voltage applied thereto. The motion of the driven crystal is amplified by a mechanically resonant system within the handpiece such that the motion of the needle connected thereto is directly dependent upon the frequency at which the crystal is driven, with a maximum motion occurring at a resonant frequency. The resonant frequency is dependent in part upon the mass of the needle interconnected therewith, which is typically vibrated by the crystal.
Power control of the phacoemulsification handpiece is highly critical to successful phacoemulsification surgery. Certain previous systems address the requirements of power control for a phacoemulsification handpiece based on the phase angle between voltage applied to a handpiece piezoelectric transducer and the current drawn by the piezoelectric transducer and/or the amplitude of power pulses provided to the handpiece. The typical arrangement is tuned for the particular handpiece, and power is applied in a continuous fashion or series of solid bursts subject to the control of the surgeon/operator. For example, the system may apply power for 150 ms, then cease power for 350 ms, and repeat this on/off sequence for the necessary duration of power application. In this example, power is applied through the piezoelectric crystals of the phacoemulsification handpiece to the needle causing ultrasonic power emission for 150 ms, followed by ceasing application of power using the crystals, handpiece, and needle for 350 ms. It is understood that while power in this example is applied for 150 ms, this application of power includes application of a sinusoidal waveform to the piezoelectric crystals at a frequesncy of generally between about 25 kHz and 50 kHz and is thus not truly “constant.” Application of power during this 150 ms period is defined as a constant application of a 25 kHz to 50 kHz sinusoid. In certain circumstances, the surgeon/operator may wish to apply these power bursts for a duration of time, cease application of power, then reapply at this or another power setting. The frequency and duration of the burst is typically controllable, as is the length of the stream of bursts applied to the affected area. The time period where power is not applied enable cavitation in the affected area whereby broken sections may be removed using aspiration provided by the handpiece or an aspiration apparatus.
As described in U.S. patent application Ser. No. 10/387,327 to Kadziauskas et al., entitled “System and Method for Pulsed Ultrasonic Power Delivery Employing Cavitation Aspects,” filed Mar. 12, 2003, discusses the beneficial aspects of transient cavitation and provides a system and method for applying energy at a level and for a time period sufficient to induce transient cavitation, and reducing applied energy after applying energy during a second nonzero lower energy period. Various ultrasonic power delivery profiles may be employed utilizing the beneficial effects of transient cavitation, including more powerful removal with reduced risk of burning or damaging the affected area.
Generally, irrigation and aspiration are employed by the surgeon using the device to remove unwanted tissue and maintain pressure within the eye. In the presence of high frequency power applications, cavitation may be generated through the needle to the unwanted tissue in an effort to break the tissue. Alternately, mechanical fragmentation may be employed using rotary, oscillatory, or reciprocating cutters to segment or grind unwanted tissue. The resultant tissue is aspirated in slurry form from the surgical site.
Issues associated with aspiration and irrigation in this environment can include difficulty in acquiring or purchasing the tissue and holding the tissue with the tip for proper removal. Use of pulsed ultrasonic power delivery, and capture and removal of unwanted tissue, particularly in high power environments where transient cavitation is encountered, can be difficult. Power delivery in the presence of relatively constant or slowly changing fluid flow characteristics can cause disruption of purchase, holding, and removal of unwanted tissue. Additionally, a mechanical cavity or cavitation cloud may form in the presence of ultrasonic cavitational energy, and such a mechanical cavity can provide a virtual barrier to efficient tissue processing. Such a mechanical cavity results from pressure differentials formed by ultrasonic energy application in the presence of a relatively constant fluid flow, and is undesirable.
Further, in the environment described, tissue may occasionally clog the lumen, or fluid passage, within the tip of the handpiece. Such clogging creates unpredictable performance characteristics and can result in undesirable pressure surges. The vacuum levels typically employed with aspiration and irrigation tend to be low to avoid clogging and other undesirable fluid flow effects. Use of low pressure can inhibit a surgeon's ability to purchase and hold unwanted tissue.
Based on the foregoing, it would be advantageous to provide an irrigation and aspiration system that effectively and efficiently operates in the presence of high ultrasonic power delivery, such as systems generating transient cavitation, and minimizes those drawbacks associated with previous tissue removal systems.